[graphite] Add the combination system's BeginBlock usages
[graphite] Add the combination system's BeginBlock usages

This is just the lowest level of the combination system. The higher
level iteration to accumulate the keys will be in a following CL.

Bug: skia:12701
Change-Id: I4015790a7bca49f11f8eb1a4fda32f235845c049
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/547819
Reviewed-by: Michael Ludwig <cb1f3dd24de0f0b611b26bc28bec758a551d9b7d@google.com>
Commit-Queue: Robert Phillips <95e8ac5504948c7bf91b22c16a8dbb7ae7c66bfd@google.com>

google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia